1. Field of the Invention
The present invention relates to a step motor control device that rotationally drives a step motor and detects the presence/absence of the rotation of the step motor, and to an electronic timepiece utilizing the step motor control device.
2. Description of the Prior Art
In an electronic timepiece, a step motor is used as a motor that rotationally drives time hands such as an hour hand or a minute hand.
FIG. 5 is a front view showing a step motor used in such electronic timepiece having an hour hand or a minute hand and described in JP57-18440 B (pgs. 1-2. FIG. 1) (hereinafter “Patent Document 1”).
In FIG. 5, the step motor includes a stator 501 made of a magnetic material, a coil 207 wound around on the stator 501, and a bipolar rotor 502 disposed within the stator 501. In the stator 501, there are saturable portions 503, 504 and inner notches 505 and 506 for determining a stop position of the rotor 502.
When a drive pulse of a rectangular wave is supplied to the coil 207 to allow a current i to flow in a direction indicated by an arrow in FIG. 5, a magnetic flux develops in the stator 501 in the direction indicated by the arrow. As a result, the saturable portions 503 and 504 are first saturated, and thereafter the rotor 502 rotates in the direction indicated by the arrow (counterclockwise) in FIG. 5 by 180 degrees due to the interactions between a magnetic pole developed in the stator 501 and a magnetic pole developed in the rotor 502. Subsequently, a pulse current different in the polarity is alternately allowed to flow in the coil 207, to thereby conduct the same operation as the above and rotate the rotor 502 counterclockwise in increments of 180 degrees.
FIG. 6 is a circuit diagram showing a conventional step motor control device for conducting the rotation control of the step motor. The circuit is structured such that a rotation drive circuit and a rotation detecting circuit are integrated together (for example, refer to Patent Document 1).
In FIG. 6, p-channel MOS transistors Q1, Q2 and n-channel MOS transistors Q3, Q4 are structural elements of the motor drive circuit, and the coil 207 of the step motor is connected between a source connection point of the transistor Q1 and the transistor Q3 and a source connection point of the transistor Q2 and the transistor Q4.
On the other hand, a detection resistor 208 connected in series with the n-channel MOS transistors Q3 to Q6 and the transistor Q5, a detection resistor connected in series with the transistor Q6, and a comparator 210 are structural elements of the rotation detecting circuit.
The gates of the respective transistors Q1 to Q6 are connected to a control circuit 103. A connection point OUT2 of the detection resistor 208 and the coil 207 and a connection point OUT1 of the detection resistor 209 and the coil 207 are connected to an input section of the comparator 210. Also, the input section of the comparator 210 is inputted with a predetermined threshold voltage Vss.
FIG. 7 is a timing chart for the case of conducting rotation control and detection control in the step motor control device shown in FIG. 6.
The operation of the conventional step motor control device structured as described above will be described with reference to FIGS. 5 to 7. First, when a drive pulse P1 is supplied to an input section Vi of the control circuit 103, the transistors Q2 and Q3 become an on-state under the control by the control circuit 103. As a result, a current flows in the coil 207 in a direction indicated by an arrow, and the rotor 502 rotates counterclockwise as shown in FIG. 5.
On the other hand, a non-detection period IT, which is a period during which the rotation of the step motor is not detected, is provided for a given period T7 immediately after the motor drive period, and a rotation detection period DT for detecting whether or not the step motor rotates is provided for a given period T8 immediately after the non-detection period IT.
In the rotation detection period DT, a rotation detection control pulse SP1 is supplied to the input section Vi of the control circuit 103. The control circuit 103 controls the on/off operation of the transistor Q4 at a given frequency in a state where the transistors Q3 and Q4 turn on in response to the rotation detection control-pulse SP1.
In this situation, a detection signal V8 is taken out from the connection point OUT1 of the rotation detection resistor 209 and the coil 207. The detection signal having a waveform shown in FIG. 7 is obtained as the detection signal V8. In FIG. 7, the detection voltage V8 lower than VDD is generated when the rotor 502 vibrates counterclockwise in FIG. 5, and the detection voltage V8 higher than VDD is generated when the rotor 502 vibrates clockwise in FIG. 5.
In the case where the rotor 502 rotates, the detection signal V8 that exceeds a given threshold voltage (Vss in this conventional example) is obtained, and a rotation detection signal of a high level is outputted from the comparator 210. In the case where the rotor 502 does not rotate, because the detection signal V8 does not reach the threshold voltage, the rotation detection signal Vs of a low level is outputted from the comparator 210. It is possible to detect whether or not the step motor rotates on the basis of the rotation detection signal Vs. After the rotation detection has been completed, the transistors Q3 and Q4 are maintained in the on-state to brake the step motor.
In a subsequent motor drive period, a subsequent normal drive pulse P1 is supplied to the input section Vi of the control circuit 103. The control circuit 103 controls the transistors Q1 and Q4 to be on, and a drive current flows in the coil 207 in an opposite direction of the above drive current (counterclockwise in FIG. 5) to thereby rotate the rotor 502 counterclockwise.
In the rotation detection period at this time, when the rotation detection control pulse SP1 is supplied to the input section Vi of the control circuit 103, the control circuit 103 controls the transistors Q4 and Q5 to be on, and controls the on/off operation of the transistor Q3 at a given frequency. In this situation, a detection voltage V is taken out from the connection point OUT2 of the resistor 208 and the coil 207, and a level of the detection voltage V is judged by the comparator 210. In the same manner as the above, in the case where the rotor 502 rotates, the rotation detection signal Vs of the high level is outputted from the comparator 210, and in the case where the rotor 502 does not rotate, the rotation detection signal Vs of the low level is outputted from the comparator 210. It is impossible to detect whether or not the step motor rotates in accordance with the rotation detection signal Vs. After the rotation detection has been completed, the transistors Q3 and Q4 are maintained in the on-state to brake the step motor.
In the step motor control device structured as described above, after the step motor has been driven by the drive pulse P1, the rotor 502 freely vibrates at a position where the rotor 502 should stop as a center. The free vibration of the rotor 502 is large immediately after the supply of the drive pulse P1 is finished, and the rotor 502 vibrates in the same direction as a normal rotation direction (counterclockwise in the above-mentioned conventional example) due to the inertia. In the case where the rotor 502 vibrates counterclockwise, the current flows in a direction indicated by an arrow in FIG. 6.
On the other hand, an equivalent circuit of the respective transistors Q3 to Q6 is made up of a series circuit comprising a switch 804 and a resistor 803, and a diode 801 and a capacitor 802 which are connected in parallel with the series circuit, respectively, as shown in FIG. 8. The respective transistors Q3 to Q6 are considered as an element equivalently having diodes in one way.
Accordingly, even though the step motor does not rotate, because the counterclockwise vibration of the rotor 502 is large within a given period immediately after the supply of the drive pulse P1 is finished, the detection voltage V7 that exceeds the threshold voltage Vss may be obtained as shown in FIG. 7. That is, in the detection signal V7 that is obtained in a given period T7 immediately after the supply of the drive pulse P1 is finished, a detection voltage having a large peak value is generated in the detection resistor 209 due to the large free vibration of the rotor 502 and misdetection is caused that the step motor is rotating.
Up to now, in order to prevent such misdetection, the control circuit has been structured such that a non-detection period IT having a given time width T7 is set immediately after the supply of the drive pulse is stopped, thereby preventing detection of the rotation of the step motor in the non-detection period IT. Accordingly, there arises such a problem that the structure of the control circuit is complicated because of the provision of the non-detection period IT.
An object of the present invention is to provide a step motor control device in which it is possible to more surely detect the rotation of the step motor with a simple structure without any provision of the non-detection period IT.
Another object of the present invention is to provide an electronic timepiece in which it is possible to more surely detect the rotation of the step motor for driving the hour hand with a simple structure.